Do you have the quest to explore the Necropolis? If so you have to go through the doors in the Plaza of Conquest (I think that's where it is - pretty sure). After you do that, than you are sent back to Cyrus and he gives you the key to open the portal.
if you did a search on the forum, this has been answered before...
and, if you read the "dialogue box" on your quest (Explore the Necropolis), you would see that is says to explore the Necropolis by entering thru the double dragon doors at the back of the Plaza of Conquests.
After you explore the Necropolis this way, then the regular portal will be open for you.
This has happened to me on my wizard too. What I did was teleport to a friend in the Necropolis, or I went through the Necropolis door in the Plaza of Conquests. After you talk to Cyrus Drake again, the portal turns on. Hope this helps!